1. Field of the Invention
The present invention relates to an artificial knee joint that will replace the knee joint.
2. Prior Art
The knee joint is replaced with an artificial knee joint when osteoarthritis, rheumatoid arthritis, or bone tumor is contracted or it is traumatized, etc. This artificial knee joint is a combination of a femoral component having two, a medial and a lateral, condyles; and a tibial component having two, a media and a lateral, articular surfaces, which support the respective two condyles. The condyles and articular surfaces must perform the same movements as with biological knee joints.
More specifically, when an artificial knee joint is attached, the balance between tension and relaxation of the ligaments that are present along the space between the femur and the tibia, particularly the collateral ligaments, needs not to be compromised; and the line that joins the lowest points on the contact surfaces between the medial and lateral condyles and the medial and lateral articular surfaces, which support these condyles so that they can freely slide (hereafter called the joint line), needs to fall inward (inclines inward) approximately 3° over the entire region of the angle of flexion-extension. If ligament balance is compromised, flexion-extension motion will be impaired; and if inward inclination cannot be maintained, adduction-abduction motion whereby the ankles turn out (abduction) during extension and turn inward (adduction) during flexion will not occur, and the same movement as with biological knee joints will not be possible.
FIG. 8A shows a generally known conventional artificial knee joint attached to a right knee and seen from the front when it is flexed 90° (flexed position), The femoral cut line BC is the same distance from the SEA (flexion-extension center axis, which is inherently horizontal) at the medial and lateral condyles, and the component thickness is the same at the medial and lateral condyles. This condylar portion can be used for both right and left knees by simple mechanization of the femoral component. Therefore, there is an advantage in that production is easy and cost is low. There is another advantage in that bone cutting during surgery is simple. However, since the joint line JL is horizontal, there is a disadvantage in that abduction-adduction motion is not induced during flexion-extension and deep flexion is not possible.
In the artificial knee joint shown in FIG. 8B, when the femur is cut, the length of this cut line BC from the SEA is set to be longer at the medial condyle than the lateral condyle, and the joint line JL is set to be inclined inward, thus being similar to a biological knee joint. However, because the medial and lateral supporting surfaces of the tibial component are at the same height, the medial condyle rises (resulting in that the SEA becomes inclined outwardly), and the medial collateral ligament on this side elongates, thus causing excess tension and making smooth flexion-extension impossible. Therefore, in some cases part of the ligament is also cut. However, when this is done, the control function of the ligament is not realized during flexion-extension and flexion-extension motion becomes awkward. In addition, bleeding occurs, prolonging surgery time and increasing the stress on the patient.
The artificial knee joint shown in FIG. 8C is disclosed in Patent Application National Publication No. 11-504226, and it is to correct the above-described problems. Excess tension on a ligament is corrected by making the medial condyle of the femoral component thinner than the lateral condyle in order to lower the medial condyle. However, although ligament balance returns to normal, there is a reduction in the degree of forward inclination of the joint line JL. Therefore, adduction-abduction motion during flexion-extension does not occur, and deep flexion is not possible. Furthermore, in this prior art, the posterior part of the medial condyle is made thinner, inducing adduction during the final stages of flexion-extension. However, forward inclination of the joint line JL must be maintained throughout all of flexion-extension. Accordingly, adduction motion, particularly, during flexion-extension does not occur and smooth flexion-extension is inhibited.